Thermal physical vapor deposition process, which is one of the processes for depositing an organic electroluminescent device, is a technique to coat an electroluminescent layer on a substrate in a housing with vaporized deposition material. In the deposition process, the deposition material is heated to the point of vaporization and the vapor of the deposition material is condensed on the substrate to be coated after the deposition material is moved out of the deposition source. This process is carried out with both deposition source holding the material to be vaporized and substrate to be coated in a vessel with the pressure range of 10−7 to 10−2 Torr.
Generally speaking, the deposition source to hold the deposition material is made from electrically resistant materials whose temperature is increased when electrical current is passed through walls (member). When the electrical current is applied to the deposition source, the deposition material inside is heated by radiation heat from the walls and conduction heat from contact with the walls. Typically, the deposition source is in the shape of box with aperture to allow vapor efflux toward the direction of the substrate.
Thermal physical vapor deposition source has been used to vaporize and deposit onto the substrate layers comprised of a wide range of materials, for example, organics of low temperature, metals, or inorganic compounds of high temperature. In the case of organic layer deposition, the starting material is generally powder. Organic powder has been recognized as giving a number of disadvantages for this type of thermal vaporization coating. First, many organics are relatively complex compounds (high molecular weight) with relatively weak bonding, and so intensive care must be taken to avoid decomposition during the vaporization process. Second, the powder form can give rise to particles of non-vaporized electroluminescent materials. The particles leave the deposition source with vapor and are deposited as undesirable lumps on the substrate. Such lumps are also commonly referred to as particulate or particulate inclusion in the layers formed on the substrate.
Further exacerbation is found in that the powder form has a very large surface area enough to support water sucked in or absorbed or volatile organics, and the volatile organics can be released during heating and can cause gas and particulates to be thrown outward from the deposition source toward the substrate. Similar considerations pertain to materials which are melted before vaporization and form droplets erupted to the substrate surface.
These unwanted particulates or droplets may result in unacceptable defects in products, particularly in electronic or optical products, dark spots may appear in images, or shorts or opens may result in failures within electronic devices.
Organic deposition apparatuses have been proposed to heat the organic powder more uniformly and to prevent the bursts of particulates or droplets from reaching the substrate. Many designs for complicated baffling structures between the source material and the vapor efflux aperture have been suggested to ensure vapor exits.
FIG. 1 is a schematic sectional view showing the inner structure of a conventional apparatus for depositing an organic electroluminescent layer, and shows a deposition source 10 mounted in a vacuum chamber 13 of the deposition apparatus and a substrate 12 located above the deposition source 10. The substrate 12 to be coated with the organic electroluminescent layers is mounted to an upper plate 13-1 of the chamber 13, and the deposition source 10 to have a deposition material 20 (organic material) is mounted on a thermally insulating structure 14 fixed to a bottom wall 13-2 of the chamber 13.
FIG. 2a is a sectional view showing the inner structure of the deposition source shown in FIG. 1, and shows that a baffle 11B is provided in the deposition source 10 to prevent particulates or droplets contained in the vapor of the deposition material 20 from directly exiting through a vapor efflux aperture 11C formed on the top plate 11A of the deposition source 10. The baffle 11B corresponds to the vapor efflux aperture 11C and is fixed to a number of support rods 11B-1 fixed to the top plate 11A of the deposition source 10 to maintain certain space from the top plate 11A.
The deposition apparatus using the deposition source 10 with the above structure has a heater or a heating means on (or under) the top plate 11A, or is constructed for the top plate 11A to have a heater in order to transfer heat to the deposition material 20 located around the center away from the side wall 11D. Thus, the heat generated at the side wall 11D as well as from the top plate 11A is transferred directly to the deposition material 20 so that the deposition material 20 is heated and vaporized. The vapor of vaporized deposition material 20 is moved along the surface of the baffle 11B and deposited on the substrate 12 (in FIG. 1) after exit through the vapor efflux aperture 11C.
FIG. 2b is a sectional view showing the change of distance between the top plate of the deposition source in FIG. 1 and the deposition material after the deposition is processed for a certain amount of time. Thus, FIG. 2b shows a state that the distance between the top plate 11A and the surface of the deposition material 20 is increased.
As explained above, the quantity of the deposition material 20 received in the deposition source 10 is decreased gradually by heating and vaporizing reactions in progressing the deposition process also the thickness of the deposition material 20 is decreased. Thus, in a certain amount of time, the initial distance (A in FIG. 2a) between the top plate 11A and the surface of the deposition material 20 in the deposition source is remarkably increased (a in FIG. 2b).
Due to increase of the distance between the top plate 11A and the surface of the deposition material 20, the heat transfer path is increased so that the deposition rate (that is, vaporization rate of the deposition material) set at the initial stage is decreased. Thus, in order to maintain the initially-set deposition rate, the temperature of the top plate 11A acting as the heater heating the deposition material 20 is needed.
In particular, while the deposition process is progressed, the distance between the top plate 11A and the surface of the deposition material 20 is increased. Under this situation, the sufficient heat generated at the top plate 11A cannot reach the deposition material 20, and so the deposition material located on the center is not vaporized though the heat generated from the side wall 11D is supplied. Consequently, if the input amount of the deposition material 20 is high (that is, the thickness of the deposition material 20 is high), it is difficult to expect that all the deposition material is vaporized.
Also, the distance between the substrate 12 and the deposition material 20, which is directly related to the uniformity of deposition layer, is increased to result in change of the deposition characteristics in time.
Low molecule organic electroluminescent material contains a large amount of organic material unstable to heat, and causes a problem of lowering the characteristics of the organic electroluminescent material by inducing resolution or change of the material characteristics due to excessive radiant heat in the deposition process. In addition, additional processes for cooling the chamber, exhausting the vacuum pressure, and re-vacuumizing are required to supply new deposition material to replenish the exhausted deposition material because the deposition process is conducted under high vacuum condition. Such additional processes cause loss of the process time.
In order to solve these problems, it is desirable to maintain uniformly the initial deposition characteristics (for example, vaporization rate of the deposition material) in supplying more deposition material in the deposition source at a time.
On the other hand, in the deposition source 10 with the structure shown in FIG. 2a and FIG. 2b, the side wall 11D acts as a heating unit (for example, structure which coils are wound around the side wall 11D). As shown in FIG. 1, however, since the sidewall 11D is exposed to the exterior, the thermal efficiency is lowered because all heat generated at the side wall 11D is not transferred to the deposition material 20 and some heat is radiated to the exterior.
In addition, as describe above, in progressing the deposition process, the deposition material 20 supplied in the deposition source 10 is consumed, and so the thickness of the deposition material 20 is decreased. Thus, heat is generated at the sidewall 11D corresponding to the portions without the deposition material and is not transferred directly to the deposition material, which contributes to energy waste.
Another drawback of the deposition source 10 is that the heat generated at the top plate 11A and the side wall 11D is not sufficiently transferred to the deposition material 20 located at the lower portion of the deposition source 10, that is, the deposition material 20 adjoining the surface of the bottom wall 11E. As a result, all of the deposition material 20 is not heated and vaporized. Particularly, depending on positions within the deposition source 10, the temperature of each deposition material 20 becomes different, that is, thermal gradient within the deposition source. Therefore, it is difficult to form a uniform deposition layer on the substrate.